The present invention relates to self-excited alternators of the brushless type, having a rotating field which is excited by an alternating current exciter whose rotating armature is mounted to rotate with the rotating field winding of the alternator and is permanently connected to the field winding of the alternator via field rectifier means also mounted to rotate with the field winding. Such a machine is referred to herein as a brushless alternator.
When a brushless alternator is running at substantially constant speed and is generating power at unity or lagging power factor, the terminal voltage of the alternator will fall with increasing load. If the terminal voltage is to be regulated so as to be kept constant at constant or slightly falling speed under load, then the direct current excitation of the field winding must be increased either with increasing load at a given power factor, or with decreasing power factor at a given load, or if both conditions occur simultaneously.
Known voltage regulation systems for brushless alternators are based upon one or the other of the two systems described in the following and referred to as "System A" and "System B".
In System A, direct current excitation requirements are controlled in response to the brushless alternator load by means of a feedback system employing the alternator terminal voltage as the only source of information indicating load change.
With System A, regulation is performed by closed loop feedback employing either a magnetic amplifier or electronic means such as SCR's or transistors capable of handling the large power requirements of transient overload excitation conditions.
Nowadays, the use of magnetic amplifiers is virtually obsolete and here it is only necessary to consider electronically controlled regulators. Electronically controlled regulators operating according to System A sense the terminal voltage which may vary with speed, load or power factor changes and, by closed loop feedback control of direct current excitation, the alternator terminal voltage is continuously restored to its correct level. Regulators of this type have the advantage of being very accurate in their steady state voltage control and in their low power consumption, but suffer from the disadvantages listed as follows:
(i) Speed of response is hampered by the magnetic response time of the exciter in addition to that of the main alternator, particularly on the removal of load or lagging power factor. This results in an over supply of excitation which prolongs and exaggerates terminal voltage overshoot. PA1 (ii) Electronic regulation systems will not always operate reliably under adverse conditions of extreme temperatures (for example between -15.degree. C. to +50.degree. C.) and/or high humidity and/or salt-laden or dust-laden atmosphere, for instance in atmosphere laden with conductive ore dust such as may be encountered in mining sites. In this respect, brushless alternators are frequently put to use in locations where extreme operating conditions prevail. Performance reliability is particularly important in such locations where it is frequently difficult to obtain trained personnel capable of servicing electronic equipment and where the brushless alternator concerned is the only means of obtaining electric power supplied at that location. PA1 (iii) Failure of the regulator generally requires immediate shut-down of the alternator to prevent the possibility of winding failure or failure of the connected load. PA1 (iv) Unless used in conjunction with additional circuitry or some external excitation supply system, electronic regulators have poorer overload characteristics than magnetically coupled systems. PA1 (i) They are large, heavy and expensive and must be mounted externally of the brushless alternator per se. PA1 (ii) They have poorer regulation than System A regulators owing to the imprecise compensation of load current, power factor, resistance changes in the exciter and main field windings, and also speed variations. PA1 (iii) Generally, they consume substantial power with the net result that alternator efficiency is noticeably reduced.
Within System B, regulation is achieved by magnetic feedback in which excitation voltage, and thus direct current excitation, are controlled in accordance with the rectified resultant of a voltage component dependent upon the alternator terminal voltage and a voltage component dependent upon alternator load current. With this system it is necessary to provide additional means for limiting the no-load terminal voltage of the alternator as well as means compensating for minor variations of temperature and/or speed variations.
One known voltage regulation system of this kind is the compounding transformer regulator which is frequently incorrectly referred to as a "saturable reactor regulator".
With the compounding transformer regulator, the primary winding of the compounding transformer is connected in series with the alternator load across the alternator terminals and the alternator terminal voltage is applied via the secondary winding of the transformer across the series combination of a rectifier and the excitation field winding of the alternator. The resistance of the excitation winding is such as to limit the direct excitation current to the value necessary for correct no-load terminal voltage. The magnitude and phase of the transformer secondary voltage is proportional to the alternator load current passing through its primary and the transformer secondary voltage combines vectorially with the alternator terminal voltage to give a resultant voltage which is rectified and applied across the exciter field winding.
System B regulators are rugged and reliable in operation and are easy to service. In addition, they have rapid response to load changes because the time constants of each magnetic section are interdependent and they provide certain protection for the alternator under overload conditions. Compared with System A regulators, they have improved characteristics for motor starting. However, System B regulators have the following disadvantages: